1. Field of the Invention
The present invention relates in general to a control technology for liquid crystal display (LCD). In particular, the present invention relates to a bias compensating driving method to minimize display flicker in LCDs.
2. Description of the Related Art
FIG. 1 is an equivalent circuit diagram of a conventional thin-film-transistor liquid-crystal-display (TFT LCD). As shown in FIG. 1, the TFT-LCD comprises scanning electrodes (or gate lines, represented as G1, G2 . . . Gx) and data electrodes (D1, D2, D3 . . . Dy). The two types of electrodes intersect with each other. Each intersection point of the scanning electrodes and the data electrodes controls an individual display unit. For example, the scanning electrode G1 and the data electrode D1 control the display unit 100. As shown in FIG. 1, the equivalent circuit of the display unit 100 comprises a thin film transistor 10, a liquid crystal capacitor Clc comprising a display electrode, a common electrode, and a storage capacitor Cs. The gate of the thin film transistor 10 is coupled to the scanning electrode G1 and the drain of the thin film transistor 10 is coupled to the data electrode D1. The data is written to the display unit 100 via the data electrodes by controlling the state of the thin film transistor 10 with the scanning electrode G1. The scan driver 3 sends scanning signals according to scanning control data to drive the scanning electrodes G1, G2, G3 . . . Gx sequentially so that only the thin film transistors of a selected scanning electrode are on at a time interval, the thin film transistors of the other (X−1) rows of the electrodes are kept off. When the thin film transistors of the selected row are on, the data driver 2 sends the corresponding video signals (Grey scale values) to the y display units on the scanning electrode via the date electrodes D1, D2, D3 . . . Dy, according to display video data. After all the x rows of the scanning electrodes are scanned and driven, the frame is completely displayed. The scanning procedure is repeated and the video signals are transmitted for the image to display.
The display frequency of a conventional LCD is about 60 Hz (60 frames per second). Each scanning electrode Gj (1≦j≦x) is scanned every 16.67 ms to allow all its thin film transistors to be sequentially activated.
The characteristics of thin film transistors are shown in FIG. 2. The time interval of t1-t3(and t3-t5) is 16.67 ms. It is assumed that VCOM is 0, and the voltage on the liquid crystal capacitor Clc is the liquid crystal display voltage VLc.
Referring to FIG. 2, in the time interval between t1˜t2, the high-level gate voltage Vgh opens all the thin film transistors on row j of the scanning electrode. The video data (a positive voltage signal with respect to VCOM at the present) is sent through the data electrodes Di (1≦i≦y) as video data (grey scale values) to the display units on row j of the scanning electrode and recharges the liquid crystal capacitor Clc of each display unit with a positive voltage. The LCD voltage VLc thus increases gradually.
At the time interval t2˜t3, the low-level gate voltage Vg1 of the scanning electrode VGj closes all the thin film transistors of the display units on row j of the scanning electrode. Because of leakage of thin film transistors, the LCD voltage VLc drops gradually toward 0V, until the temporal point t3 when the LCD voltage VLc reaches a first voltage value V1.
At the time interval of t3˜t4, the high-level gate voltage of the scanning signal VGj opens all the thin film transistors on the display units on row j of the scanning electrode. The display video data (now being negative voltage signal with respect to VCOM) is sent as the video signal (Grey scale values)through the corresponding data electrodes Di (1≦i≦y) to the corresponding display units on row j of the scanning electrode and recharges the LCD capacitor Clc with a negative voltage. Consequently, the LCD voltage value VLc is negative with increased value. At the interval t4˜t5, the low-level gate voltage Vg1 of the scanning signal VGj closes all the thin film transistors of the display units on row j of the scanning electrode. The LCD voltage VLc, due to leakage of the thin film transistors declined toward 0V to become a second voltage value V2 at the timing point t5.
The time interval t2˜t3 is usually referred to as the positive field period, and the interval t4˜t5 is referred to as the negative field period. The leakage current of the thin film transistors are different in the positive and the negative field periods because of the voltage differences between the gate and the source Vgs at the two periods. Thus, the first voltage value V1 and the second voltage value V2 are different. With respect to the root-mean-square, rms of the LCD voltage, VLc, the rms of the LCD voltages during the positive and the negative filed periods, the difference between the two results in the change in light transmittance. The result is display flicker with a frequency of 30 Hz.